As bonding means with high workability and high adhesion reliability, pressure-sensitive adhesive tapes are used for application of fixing parts and application of labels that indicate product information in the various industrial fields of OA equipment, home electric appliances, automobiles, and the like. On the other hand, from the viewpoint of protection of the global environment, assembly products such as OA equipment are increasingly disassembled after use, recycled, and reused. In this case, when parts are bonded together with pressure-sensitive adhesive tapes or labels are attached to parts, the work of removing the adhesive tapes or labels is required, and the property (so-called removability) of being capable of being removed without leaving adhesive residue or breaking support base materials is required. Further, in recent years, the suppression of emission of volatile organic compounds (so-called VOC) has been strongly demanded. For the pressure-sensitive adhesive tapes, solventless-type pressure-sensitive adhesives have attracted attention, and substitution of solvent-type pressure-sensitive adhesives with aqueous dispersion-type pressure-sensitive adhesives has been demanded.
A pressure-sensitive adhesive double coated tape so far disclosed as a pressure-sensitive adhesive double coated tape having high adhesion reliability and removability includes a nonwoven fabric core having specified strength; and an acrylic pressure-sensitive adhesive composition containing, as a main component, an acrylic copolymer which is composed of (a) an alkyl(meth)acrylate having an alkyl group with 1 to 12 carbon atoms, (b) a nitrogen-containing vinyl monomer, and (c) a hydroxyl group-containing monomer or carboxyl group-containing monomer, which is reactive to a crosslinking agent, and (d) an isocyanate-based crosslinking agent (refer to Japanese Unexamined Patent Application Publication No. 8-209086). However, when the pressure-sensitive adhesive double coated tape of this technique is formed by an emulsion polymerization method, it is difficult to exhibit excellent removability.
Also, a pressure-sensitive adhesive sheet using an emulsion-type pressure-sensitive adhesive and having both adhesion reliability and removability is disclosed (refer to Japanese Unexamined Patent Application Publication No. 2006-265537). As means for imparting both adhesion reliability and removability, a technique for an acrylic pressure-sensitive adhesive composition is disclosed, in which acrylic acid and methacrylic acid are combined in a total amount of 1.5 to 2.5% by mass, the ratio by mass of methacrylic acid to acrylic acid being 0.5 to 2.0. However, when a pressure-sensitive adhesive double coated tape having higher adhesive strength is prepared, it is difficult to secure satisfactory removability.
In addition, with respect to an emulsion-type pressure-sensitive adhesive characterized by containing acrylic acid and methacrylic acid, a technique for an acrylic pressure-sensitive adhesive composition is disclosed, in which acrylic acid and methacrylic acid are combined at a ratio by weight of 1/1 to 1/5, the content of acrylic acid and methacrylic acid being 2 to 10% by weight (refer to Japanese Unexamined Patent Application Publication No. 2005-248059). However, the influence of combination of acrylic acid and methacrylic acid on removability is not described. In addition, when the acrylic pressure-sensitive adhesive disclosed in this patent document is used as a component, adhesive force greatly increases with time after attachment, and thus it is difficult to secure removability. In particular, there is a problem that adhesive force to a metal surface such as stainless steel or the like greatly increases.
Further, when an adhesive tape using an emulsion-type adhesive is used for fixing a high-air-permeability member such as an urethane foam, a nonwoven fabric, or the like, there occurs a problem that the pressure-sensitive adhesive double coated tape peels off from the high-air-permeability member in an environment of high humidity.
A problem to be solved by the invention is to provide an aqueous dispersion-type emulsion pressure-sensitive adhesive capable of forming a pressure-sensitive adhesive tape which strongly adheres to adherends, causes no excessive increase in adhesive force in long-term adhesion, and can be removed without using a special treatment such as heating or the like and without leaving adhesive residue or breaking a support base material.
Further, a problem is to provide an aqueous dispersion-type emulsion pressure-sensitive adhesive capable of forming a pressure-sensitive adhesive tape which is used for fixing high-air-permeability members such as an urethane foam, a nonwoven fabric, and the like, and which does not peel off even in an environment of high humidity and is thus excellent in water resistance.